Some notes I made for myself sometime ago... up to you what you do with them...
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Open In Scheduler
This dropdown appears across many “project pages” – can not affect it by data (eg by nulling the scheduler format column on the database).
File projectPageToolbar.xsl located in directory{NIKU_HOME}\META-INF\projmgr\vxsl\portlets\projects\ contains the code that builds the page.
Comment out (or remove) the lines;
<xsl:call-template name="projectSchedulerLaunchActions">
<xsl:with-param name="can_view" select="$can_view_mgt"/>
<xsl:with-param name="readOnly" select="not($can_copy)"/>
</xsl:call-template>
And restart the APP (or just flush the 'VXSL' groups cache)
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I think that that removes the options entirely though, so if you still want Gantt then you'd need to experiment.